In the following years Siodmak wrote many novels, screenplays, and short stories including the novel F.P.1 antwortet nicht (F.P.1 Doesn't Answer) (1932) which was adapted into a film featuring Hans Albers and Peter Lorre.

Siodmak decided to emigrate after hearing an anti-Semitic tirade by the Nazi propaganda minister Joseph Goebbels, and departed for England where he made a living as a screenwriter before moving to the United States in 1937. His big break in Hollywood came with the screenplay for The Wolf Man (1941), starring Lon Chaney, Jr., which established this fictional creature as the most popular movie monster after Dracula and Frankenstein's monster.[1] In the film, Siodmak created several werewolf "legends": being marked by a pentagram; being practically immortal apart from being struck/shot by silver implements/bullets; and the famous verse:

Even a man who is pure in heart,And says his prayers by nightMay become a Wolf when the Wolfbane bloomsAnd the autumn Moon is bright

(The last line was changed in the sequels to "And the Moon is full and bright".)